1. Field of the Invention
This invention relates to a plastic-clad silica optical fiber, and a method for producing the same. More particularly, this invention relates to an optical fiber having a core-clad structure wherein the core material is transparent inorganic glass and the clad material is a transparent organic polymer, and to a method and apparatus for producing the same.
2. Description of the Related Art
Optical fibers having a core-clad structure and consisting of a pure silica or glass core and a plastic cladding (hereinafter referred to as PCS fibers) have been known, and they have hitherto been used in various fields, such as in the field of light communication over short distances of not more than 5 km, as light guides, as optical fibers to transmit images or energy, by making use of large apertures, large numerical apertures and low transmission loss properties.
Clad materials for PCS fibers should have 1) a low refractive index, 2) high transparency and 3) good adherence to a core material. Examples of those clad materials include heat-curable or light-curable silicone resins and fluorine-containing resins. However, it is difficult to find a crosslinking clad material which satisfies all three of the above requirements. A clad material having enough heat resistance and satisfying all three requirements has never been developed.
PCS fibers having a clad material of a tetrafluoroethylene-hexafluoropropylene copolymer or tetrafluoroethylene-vinylidene fluoride copolymer have been reported (Japanese Patent Kokoku Publication No. Sho. 48-25665 and Japanese Patent Kokai Publication No. Sho. 51-52849), but these clad materials often become opaque because of their crystalline nature, and thus the transmission loss of PCS fibers using these clad materials often becomes great.
Regarding the methods for coating a silica core with a clad material, methods such as 1) applying a liquid clad material onto an inorganic core, and then crosslinking the liquid clad material; 2) melt-covering a core with a clad polymer; 3) applying a solution of a clad polymer and a solvent onto a core fiber and evaporating the solvent, are generally employed.
However, a PCS fiber which is produced by one of the above methods has its physical properties changed over time, and this change over time becomes a serious problem for a PCS fiber that must have a light transmission loss of not more than 10 dB/km.